Add unit tests for untested SkString APIs.
Add unit tests for untested SkString APIs.

Unit tests for SkString::appendVAList would have preemptively caught the
associated fuzzer failure.

Change-Id: I19a414e5e937f9e3fbe0f75e062b4befa6e2877b
Bug: chromium:1092743
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/297473
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Commit-Queue: Greg Daniel <39df0a804564ccb6cf75f18db79653821f37c1c5@google.com>
Reviewed-by: Greg Daniel <39df0a804564ccb6cf75f18db79653821f37c1c5@google.com>

google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia